1. Field of the Invention
The present invention relates to a production process of a light amount adjustment member, by which a light amount adjustment member which can be usefully used in a wide variety of fields such as optical instruments such as digital cameras and video cameras, and electrophotographic recording apparatus can be provided with ease, a light amount adjustment member obtained thereby, a light amount adjustment device, and a photographing apparatus.
2. Related Background Art
In an image forming (photographing) optical system used in an optical instrument such as a camera, is generally contained a light amount adjustment device that controls light quantity of an incident beam, i.e., the so-called diaphragm blade device. In such a diaphragm device, a plurality of diaphragm blades form an opening of a prescribed area, and the quantity of a beam passing through the opening is controlled by controlling an opening diameter of the opening by an actuator. However, an influence of diffraction caused at an end of the diaphragm blade becomes great as the opening diameter of the opening is made small, so that the image forming performance of the image forming optical system is deteriorated. On the other hand, there has been known a technique that a filter as a light amount adjustment member is provided at a part of the diaphragm blade in order to avoid this defect, thereby attenuating the quantity (light quantity) of a bundle of rays passing through the opening by the optical filter instead of making the opening diameter small. The optical filter used for such purposes is required to lessen optical defects such as light scattering, refraction abnormality and spectral transmittance deviation.
That generally used as the light amount adjustment member in the past is of the type that a light-absorbing coloring material such as a pigment or dye is mixed and incorporated into a transparent film-forming material to form a colored film. However, light amount adjustment members produced by this method have been very expensive and have not sufficiently met cost reduction required of increasing demand. In addition, the method that the coloring material is incorporated into the transparent film-forming material to form the colored film has been markedly difficult to produce a light amount adjustment member having a continuous or stepwise density distribution (hereinafter referred to as “multi-density”).
Another known production method of the light amount adjustment member includes a method of a multi-density light amount adjustment member by using a silver halide film (see Japanese Patent Application Laid-Open No 5-173004). However, the light amount adjustment member obtained by this method causes a problem characteristic of the use of the silver halide film that the rectilinearity of rays passed through a filter is impaired by reflection of a beam on the surfaces of silver particles contained in the filter and diffraction of a beam passed through ends of the silver particles to deteriorate the image forming performance of the optical system.
A method of producing a multi-density light amount adjustment member by a vapor deposition method is also disclosed (see Japanese Patent Application Laid-Open No. 10-133254). However, this method incurs increase in production cost and is thus expensive. In addition, since the film thickness thereof varies according to the density, a problem that a difference in film thickness arises between a high-density portion and a low-density portion, and consequently, an optical path difference arises to deteriorate resolution. Further, according to this method, a light amount adjustment member which the density distribution varies stepwise can be produced, but that having a continuously varied density distribution is considerably difficult to produce.
There has also been proposed a process of producing a multi-density light amount adjustment member by first incorporating organic coloring matter which fades by light in a film forming material, and partially irradiating the resultant film with high-energy light, thereby decomposing the organic coloring matter at irradiated portions (see Japanese Patent Application Laid-Open No. 10-96971). However, according to this process, usable coloring materials are limited to those which fades by light. It is therefore very difficult to obtain a product having sufficient optical properties. In addition, it may be easily inferred that resulting products will become expensive because of the complicated production process.
It is further disclosed to form a film having a single density in the form of halftone dots by vapor deposition or a printing process such as a photomechanical process and to vary a halftone dot pattern from position to position, thereby producing a filter as a light amount adjustment member that a transmittance varies steplessly (see Japanese Patent Application Laid-Open No. 2000-352736). However, in such a process, a film of a prescribed density is formed by the photomechanical process or vapor deposition. Even if either process is adopted, a problem that the apparatus becomes large and expensive, and so the production cost of the filter becomes high.